1. Field of the Invention
This invention relates to apparatus for separating contaminants from the exhaust gases for emission of contaminant-free exhaust gases to the air, by lowering and controlling the exhaust gases to the required ambient temperature to facilitate removal of the contaminants while recovering useful heat.
2. Description of the Prior Art
Flue gas recovery devices are well known and, in particular devices associated with power generation plants and coal, oil or gas-fired boilers for removing from the flue gas stream NO.sub.x and SO.sub.x emissions. An example of conventional devices that not only recover heat from flue gas but also remove the contaminants is disclosed in U.S. Pat. No. 4,121,541 where a flue gas from a power generating plant is purified and heat is recovered from the flue gas. The flue gas from the boiler initially enters a heat exchanger where it is cooled and thereafter contacted with cooling water to remove waste heat. Pollutants contained in the flue gas are partially absorbed by cooling water. Thereafter the purified flue gas is released with ambient cooling air into the surrounding atmosphere. This system is characteristic of a wet flue gas scrubber used to remove contaminants in the form of sulfur dioxide, fluorine compounds, nitrogen oxides, etc.
Other known devices for removing contaminants from exhaust gases emitted from power plants are disclosed in U.S. Pat. Nos. 3,473,298 and 4,799,941. With these devices the exhaust gases are first chilled with direct water sprays and thereafter solid contaminants and water soluble substances are removed from the gases by the contacting water. In a spray chamber the water combines with water soluble gases, such as SO.sub.x, contained in the gases to form sulphurous and sulphuric acids which are collected with the water spray in a chamber. It is also known as disclosed in U.S. Pat. No. 3,881,004 to recover nitric acid by scrubbing a tail gas with acid or alkaline solution in nitric acids plants which minimize the discharge of nitrogen oxides to the atmosphere.
While heat recovery devices or economizers are well-known, their efficiency needs to be improved because it has been determined that up to 16% of heat escapes unrecovered from the stacks equipped with economizers. This in part can be attributed to emitting the exhaust gas at a relatively high exit temperature. Also, in the case of "mass transfer" heat recovery devices the primary function is to recover heat. Generally the removal of the contaminants from the flue gas stream is incidental. When scrubbing operations are combined with heat removal operations, the process becomes less efficient at heat transfer because a portion of the heat flow is lost in the effort to remove contaminants.
It is generally recognized that wet scrubbers are designed primarily to remove contaminants and are not efficient in recovering waste heat. Heat removal is considered incidental and adds to the expense of the pollution abatement operation. In many instances after wet scrubbing, flue gas must be reheated to rise up and out of the boiler chimneys. As a result boilers equipped with gas scrubbing equipment are expensive to maintain and to operate.
There is need to provide apparatus for removing contaminants from exhaust gases that allows for the efficient use of recovered waste heat. Differently from power generation equipment there are many industrial and commercial contaminating sources which are inherently producing exhaust gases at lower temperature which are not readily adaptable to known prior art devices for waste heat recovery. Therefore there is need for apparatus that allows for the efficient extraction of heat from exhaust gases as they are being treated for the removal of contaminants.